As one technique for the GC analysis, a technique called the “comprehensive two-dimensional GC” (which is also called “GC×GC”) is commonly known (for example, see Patent Literature 1 and Non Patent Literature 1).
In the comprehensive two-dimensional GC, various components contained in a sample are initially separated with a column which is the first dimension (which is hereinafter called the “primary column”). The thereby eluted components are introduced into a modulator. The modulator repeats an operation including the steps of catching the introduced components at regular intervals of time (which is normally within a range from a few seconds to approximately one dozen seconds; this interval of time is normally called the “modulation time”), detaching those components with an extremely narrow time bandwidth, and introducing them into a column which is the second dimension (which is hereinafter called the “secondary column”). In general, the component separation in the primary column is performed under such a separation condition that the elution occurs at a rate approximately equal to or slightly lower than the rate in a commonly used GC. On the other hand, as compared to the primary column, the secondary column has a different polarity, shorter length and smaller inner diameter, with the component separation performed under such a condition that each elution will be completed within the modulation time. In this manner, in the comprehensive two-dimensional GC, a plurality of components which have not been separated by the primary column and whose peaks overlap each other can be separated in the secondary column, whereby the separation performance is dramatically improved as compared to normal GCs.
A similar technique to the comprehensive two-dimensional GC is also known in the field of liquid chromatographic analysis, i.e. the comprehensive two-dimensional LC or LC×LC, which uses two columns having different separation characteristics. In the present description, both the comprehensive two-dimensional GC and the comprehensive two-dimensional LC are collectively called the “comprehensive two-dimensional chromatograph”.
These comprehensive two-dimensional chromatographs detect the components in a sample gas or sample solution which has passed through the two columns. Therefore, the data produced by the detector is a single sequence of data arranged in time-series order. Accordingly, by plotting the obtained data in order of generation, a one-dimensional chromatogram as shown in FIG. 3A can be created, which is similar to a chromatogram obtained with a normal GC, i.e. which has the horizontal axis indicating the retention time and the vertical axis indicating the signal intensity. In FIG. 3A, “tm” denotes the modulation time. The section of the chromatogram within this time (tm) is the chromatogram which reflects the state of separation of the components in the secondary column.
As noted earlier, in many cases, the two columns in the comprehensive two-dimensional chromatograph have different separation characteristics. Therefore, to show the state of separation in each column in an easily comprehensible form, a two-dimensional chromatogram having two orthogonal axes which respectively indicate the retention time in the primary column and the retention time in the secondary column is created, with the signal intensity represented by contour lines, color scale, or gray scale. FIG. 3B illustrates the order in which the data are arrayed to create the two-dimensional chromatogram from one-dimensional chromatogram data. The range of the vertical axis of this graph corresponds to the modulation time, tm. The one-dimensional chromatogram data are sequentially plotted upward from the lower end (0) along the vertical axis (the solid arrow in FIG. 3B). After reaching tin, the plotting point is shifted rightward along the horizontal axis and returned to the lower end of the vertical axis (the broken line in FIG. 3B). After that, the upward-plotting operation along the vertical axis is once more performed. By repeating such an operation, a two-dimensional chromatogram as shown in FIG. 3C is obtained. In FIG. 3C, the signal intensity is indicated by contour lines.
In the case of a temperature-programmed analysis in which the temperature of the columns is increased with time, the horizontal axis in the two-dimensional chromatogram represents the order of the boiling point, while the vertical axis represents the order of polarity. Therefore, the analysis operator can easily understand the nature of each component on the basis of the two-dimensional chromatogram. Even when many components contained in the sample, the analysis operator can intuitively understand what kinds of components are contained.
As a data processing software product for creating such a two-dimensional chromatogram, the “GC Image” offered by GC Image LLC is commonly known.
As noted earlier, the comprehensive two-dimensional GC provides a high level of separation power, and therefore, is extremely effective for an analysis of a sample which contains a number of compounds whose retention times are close to each other, a typical example of which is a hydrocarbon analysis of diesel fuel. In particular, in such an application area, the device is commonly used in comparative analyses for various purposes, such as the discrimination of similar articles, identification of causative substances for malfunctions (or the like), or analysis of temporal changes. In order to facilitate such comparative analyses, conventional data-processing software products for comprehensive two-dimensional GCs are provided with various functions, such as the comparison of two-dimensional chromatograms, numerical comparison of the blobs detected on the two-dimensional chromatograms, as well as multivariate analysis (see Non Patent Literatures 2 and 3).
In general, the aforementioned types of comparative analyses often include the task of comparing a plurality of two-dimensional chromatograms. If there is an extremely large number of blobs on the chromatograms, the analyzing efficiency becomes low since a considerable amount of time is required for the tasks of finding a difference between the chromatograms, choosing notable blobs or compounds, and determining the correspondence between the blobs and the result of another analysis performed for the sample. No appropriate function for assisting such tasks performed by the analysis operator is provided in the conventional data-processing software products for two-dimensional GCs.